Timing relations to influence gender

In general, a couple may delay relations when there is mutual consent.  This could be one such case.

The mitzvah of onah (marital relations) is formally an obligation of husband to wife at a set interval (onah), and is discussed in Even HaEzer 76. On any given onah, the wife is permitted to waive her right to relations, so long has the couple has fulfilled the Torah's mitzvah of peru urvu, having one boy and one girl. In your case, the prospect is a delay and not a waiver, and the purpose is presumably to attempt to fulfill peru urvu.  The Talmud in Niddah 31a-b indicates permissibility of attempting to plan a child's gender, even by adjusting relations.

Additionally, nowadays, most of the specific timings discussed there are no longer binding.  (On this last point, see for example Igrot Moshe III:28 for a discussion of the mitzvah of onah.)

If the night in question is mikveh night, there is still room to permit a delay, on the logic that mikveh night works like a regular onah (see Igrot Moshe above).  In fact, when a veset comes out on mikveh night, most opinions hold that relations are delayed.  If you are particularly concerned about missing the mitzvah of relations on mikveh night, you could consider delaying immersion until the night on which you would like to attempt conception.

Please note, however, that the efficacy of this type of timing effort has not been medically established.